Photovoltaic cell including wavelength shifter comprising lanthanide chelate fluorophores based on dihydropyridine condensation products

ABSTRACT

Photovoltaic cell and wavelength-shifting device, comprising a dihydropyridine condensation product chelated to a lanthanide metal ion, to expand the solar spectrum available to the cell for conversion into electricity. Method to detect an amine or an aldehyde for forming a dihydropyridine condensation product chelated to a lanthanide metal ion and measuring the long life fluorescence of the chelated metal ion to determine the amount of amine or aldehyde present.

BACKGROUND OF THE INVENTION

This invention relates to useful applications of the formation of adihydropyridine condensation product formed by the reaction of a-diketone, an aldehyde and an amine. More particularly it relates to awavelength shifting device which permits a photovoltaic cell to collectenergy from the energy-rich portion of the solar spectrum and tosensitive methods of detecting amines and aldehydes. The inventionexploits the long-lived fluorescence and large Stoke's shift associatedwith the lanthanide ion chelate fluorophores that can be made with thecondensation products.

That part of the solar spectrum below 450 nm is poorly or not at allavailable for conversion to electricity by photovoltaic cells.Furthermore, this part of the solar spectrum is very rich in energy atthe surface of the earth and even more so extraterrestrially. Thesefacts are well documented in "Sunlight to Electricity: Prospects forSolar Conversion by Photovoltaics"Joseph A. Merrigan, MIT Press,Cambridge, Mass. (1975). Attempts have been made to reduce the problemusing luminescent solar collectors (LSC) which are dye-doped plastics orglass plates. A type of dye advocated is a weak metal chelate. Forexample, M.S. Cook and A.J. Thomson in "Chemistry in Britain"(October1984, p.914-917) advocate the use of ruthenium (II) complexes with2-2'-bipyridine or 1-10-phenanthroline. They report, however, that thesematerials do not have long term photostability. This problem stems fromthe low stability that would be associated with the use of a metal tobidentate chelate even in 1:3 ratio in dilute solution in the plastic orglass. What is required is a fluorophore with a large Stoke's shiftwhich is also able to remain in long term photostability.

It is known from U.S. Pat. No. 3,956,341 and International PatentApplication PCT/GB85/00337 that an aldehyde (R¹ --CHO), an amine (R²--NH₂) and a β-diketone (R³ COCH₂ COR⁴) (with R¹, R², R³ and R⁴ beingarbitrary organic radicals and R¹ and R² optionally being hydrogen)react to form a dihydropyridine condensation product as illustrated inFIG. 1. The reaction is preferably carried out at a mildly acidic pH(5.5-6.5) and a mildly elevated temperature (30°-80° C.). It isdependent only on the basic structure of the aldehyde, amine andβ-diketone and not on the nature of the substituents R¹ to R⁴ so that itcan be used with a wide variety of aldehydes, amines and β-diketones.Examples of β-diketones are trifluoroacetylacetone,thenoyltrifluoroacetone and benzoyl and alpha- and beta-naphthoyltrifluoroacetone as well as the β-diketones mentioned in U.S. Pat. No.4,374,120. Other β-diketones that might be employed are carboxy-modifiedversions of the above mentioned β-diketones.

The dihydropyridine condensation product, according to Nash (T. Biochem.J. 55, 1953, p. 416-421), has the capability to form an enol at the 4position and probably holds a metal ion by chelation at that site. Whenthe chelated metal is a lanthanide metal ion, especially Eu(III) orTb(III) but also Sm(III) or Dy(III), the metal ion and the condensationproduct exist as an acceptor-donor pair, so that the condensationproduct acts as a chelating chromophore, absorbing excitation radiationat its characteristic absorption peak(s) and by energy transfer inducingthe resonance fluorescence of the lanthanide metal ion. Thesefluorescence properties are recognized in International PatentApplication No. PCT/GGB85/00337 and used to produce lanthanide ionfluorescent labels to be used in fluoroimmunoassays. The valuableproperties of the chelating condensation products can be utilized inseveral ways and the present invention is concerned with theutilization.

Any chelates with suitable absorption and donor properties as thosedescribed for the condensation product and able to form kineticallystable 1:1 chelates with lanthanide metal ions would serve a similarpurpose of wavelength conversion. A class of such chelates and methodsfor making them are disclosed in European Patent Application No.0,195,413. Those with good quantum efficiencies for the fluorescence ofEu(III), Sm(III) and Dy(III) are to be preferred as the principalemission bands, as shown in FIG. 3, of these ions are more available tothe commonest sort of photovoltaic cells. Except for CdS cells, theabsorption edges for most other popular photovoltaic materials liebeyond 800 nm.

In addition to the work that has been done on photovoltaic cells anddihydropyridine condensation products, fluorometric methods have beenused to detect chemical substances. The sensitivity of these detectionsystems is inhibited by the high background fluorescence associated withmost organic substances. A highly sensitive analytical procedure for thedetermination of formaldehyde, for example, is useful in the study ofbiological systems and air pollution. Many biological substances such assugars, hydroxamino acids, methanol, formic acid etc. are determined byfirst converting them to formaldehyde by oxidation or reduction. Also,the detection and estimation of amines, especially in amino acids andproteins, are important in biochemical studies. In chromatography it isimportant to be able to detect small quantities rapidly. The use ofchelates of lanthanide ions as fluorescent labels in the determinationof aldehydes and amines offers a great improvement in signal to noiseratios over previously used fluorophores.

SUMMARY OF THE INVENTION

In accordance with the present invention, it has been discovered thatphotovoltaic cells, when optically coupled to a wavelength-shiftingdevice comprising a polymer containing a lanthanide metal ion chelatedto a dihydropyridine condensation product, can utilize energy from theenergy-rich portion of the solar spectrum. This portion of the solarspectrum was previously unavailable to photovoltaic cells. Accordingly,the present invention utilizes a dihydropyridine condensation productwhich is chelated to a lanthanide ion in a polymer as awavelength-shifting device which can be coupled to a photovoltaic cell.The condensation product absorbs a significant level of energy andtransfers the energy to the lanthanide ion. The lanthanide ion thenemits or fluoresces at a longer wavelength. Present photovoltaic cellsare only able to collect energy from these longer wavelengths. Bycoupling this chelated condensation product to a solar cell, energy fromthe low-wavelength portion of the solar spectrum is finally availablefor conversion to electricity.

The present invention also uses the components of the dihydropyridinecondensation product chelated to a lanthanide ion to detect for aminesand aldehydes. The dihydropyridine condensation product is composed of aβ-diketone, an aldehyde and an amine. It exploits the long-livedfluorescence and large Stoke's shift associated with the lanthanide ionchelate fluorophores that can be made with the condensation products

It is therefore an object of the present invention to provide aphoto-converting device which absorbs energy from the energy-richportion of the solar spectrum (280-460 nm).

It is another object of the present invention to provide a coating forpresent photovoltaic cells which absorbs energy from the energy-richportion of the solar spectrum (280-460 nm) and emits energy in a form(540 nm and beyond) which is better available to present photovoltaiccells.

It is a further object of the present invention to provide a coating forpresent photovoltaic cells which absorbs energy from the energy-richportion of the solar spectrum (280-460 nm) and emits energy in a form(540 nm and beyond) which is better available to present photovoltaiccells and the fluorophore has long term photostability.

It is another object of the present invention to increase thesensitivity or signal to noise ratio over previously used fluorophores.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an illustration of the dihydropyridine condensation reaction.

FIG. 2 is a schematic diagram of the photo-electric device with acoating containing a lanthanide metal ion chelated to a dihydropyridinecondensation product.

FIG. 3 shows the principal emission lines of some of the lanthanide ionsof interest.

FIG. 4 illustrates the slightly modified moleculethenoyltrifluoroacetone which provides good solubility and extracoordination ability.

FIG. 5 illustrates an example of a chelating aldehyde and a method forproducing it.

FIG. 6 illustrates the excitation spectrum of the β-diketone shown inFIG. 4 chelating Eu(III) with an emission of 613 nm.

FIG. 7 illustrates the excitation spectrum of the dihydropyridinecondensation product of the β-diketone of FIG. 4 and the chelatingaldehyde of FIG. 5 with a mouse monoclonal antibody againstcarcinoembryonic antigen (CEA) as an example both of an amine and apolymer. Eu(III) has an emission of 613 nm.

FIG. 8 illustrates the excitation spectrum of multiple covalently linkedβ-diketone of FIG. 4 chelating Eu(III).

FIG. 9 illustrates the excitation spectrum of the condensation productof the multiple-covalently linked β-diketone, the chelating aldehyde ofFIG. 5 and the monoclonal anti CEA. Eu(III) has an emission of 613 nm.

FIG. 10 illustrates the excitation spectrum of the condensation productof the multiple-covalently linked β-diketone, formaldehyde and themonoclonal anti CEA. Eu(III) has an emission of 613 nm.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

At the outset, the invention is described in its broadest overallaspects with a more detailed description following. The properties ofthe condensation product are utilized to produce a wavelength-shiftingmaterial. Such a material comprises the chelation product of afluorescing lanthanide metal ion and a solid forming dihydropyridinecondensation product of an NH₂ -bearing reagent, a β-diketone-bearingreagent and an aldehyde-bearing reagent, one of which reagents formspart of a polymer. The material absorbs energy at the excitation bandabsorption maxima of the dihydropyridine condensation product and emitsenergy at the characteristic wavelength corresponding to the metal ionschelated.

Conveniently, the NH₂ -bearing reagent is a polymer. Polyethyleneimineis an example of such a polymer. Alternatively the aldehyde-bearingreagent may be a polymer. If desired, pre-existing polymers used asstructural parts of conventional photovoltaic cells may be modified byincorporation of amine or aldehyde or β-diketone groups prior toreaction with the other reagents to form the dihydropyridinecondensation product. Such a polymer is the polyimide used with CdSphotovoltaic cells as described by S. A. Merrigan in "Sunlight toElectricity: Prospects for Solar Conversion by Photovoltaics."MIT Press,Cambridge, Mass. (1975).

The wavelength-shifting material may be dispersed in a transparentmaterial such as glass or a plastic material, for example polystyrene orpolypropylene or some co-polymer suitable for use with photovoltaiccells, if it does not itself already form a transparent polymer usefulfor such purposes. The resulting product may be placed on one face of asilicon or other photovoltaic cell or as a part of a luminescent solarcollector device (LSC). Such a device will absorb solar energy at thewavelengths of 280-460 nm, which is the energy-rich region of the solarspectrum. This energy range is generally not available to conventionalphotovoltaic devices. The device will emit energy in the region of540-650 nm and beyond, which is a range that is better available tosilicon or other photovoltaic cell or device. Further details of theconstruction and arrangement of an LSC device are given in an article byM. S. Cook and A. J. Thomson. Similar constructions can be used with thecharacterizing condensation products of this invention.

Any chelates with suitable absorption and donor properties as thosedescribed for the condensation product and able to form kineticallystable 1:1 chelates with lanthanide metal ions would serve a similarpurpose of wavelength conversion. A class of such chelates and methodsfor making them are disclosed in European Patent Application No.0,195,413. Those with good quantum efficiencies for the fluorescence ofEu(III), Sm(III), and Dy(III) are to be preferred as the principalemission bands of these ions are more available to the commonest sort ofphotovoltaic cells. Except for CdS cells, the absorption edges for mostother popular photovoltaic materials lie beyond 800 nm.

The formation of the condensation product is used to present a metal ina convenient form, possibly having new electronic, electrical orchemical properties. Because it is possible to use a wide variety ofamines and aldehydes in the condensation reaction, the choice of thesematerials can be dictated by a desire to chelate any desired metal. Whenthe metal-bearing condensation product is formed in a polymer ordispersed in a polymer, the metal is present in a kinetically stablestate as a result of its strong chelation. Thus it is possible toprovide a thin film of one or more metals by this means.

The photoelectric device, illustrated in FIG. 2, consists of aphotovoltaic cell (1) with a coated polymer (2) on its face. The coatedpolymer (2) is a material with long term photostability. The coatedpolymer (2) is a polymer of a dihydropyridine condensation productchelated with a lanthanide metal ion. The coated polymer (2) is composedof the reactants illustrated in FIG. 1 and reacted under the conditionspreviously mentioned. A suggested β-diketone for this purpose is themodified thenoyltrifluoroacetone illustrated in FIG. 4 which increasessolubility and increases the stability constant for chelating lanthanidemetal ion in the final dihydropyridine compound. This β-diketoneprovides extra coordination ability. An example of a chelating aldehydeand a method for producing it is illustrated in FIG. 5. The chelatedlanthanide ions which are chelated to the condensation product emitenergy within the range most accessible to the photovoltaic cell and arelisted in FIG. 3. The coating absorbs energy from the energy-richportion of the solar spectrum (280-460 nm) as illustrated in FIG. 7. Thecoated polymer is then absorbed on a transparent material (3) and placedon the face of the photovoltaic cell. The device as shown in FIG. 2 inits entirety is subjected to sunlight and converts energy intoelectricity.

The following reagents were used in the following example which ispresented for illustrative purposes only and is not intended to limitthe scope of the invention.

(1) A commercially available polymer which possesses an amino group,mouse monoclonal antibody against carcinoembryonic antigen (CEA), is ina solution containing approximately 10 mg/ml.

(2) A 1:1000 dilution of a aldehyde solution originally 37-40% w/v inthe aldehyde of FIG. 5, thus now having a concentration of about1.4×10⁻³ moles/liter.

(3) A stock solution of the modified β-diketone illustrated in FIG. 4 inmethanol at a concentration of 160 mM.

EXAMPLE

The coating was manufactured with 400 μl of the mouse monoclonalantibody solution (2.7×10⁻⁸ mole of antibody) were incubated at 37° C.for 1 hour with 200 μl of the diluted aldehyde solution (2.7×10⁻⁷ moleof aldehyde) and 4 μl of the β-diketone solution (5.4×10⁻⁷ mole ofβ-diketone) in an acetate buffer (0.2M) at a pH of 5.7. The reactionproduct was dialysed initially against the acetate buffer to removeunreacted small molecules then against the acetate buffer containingEu(III) ions at 10⁻⁷ M concentration to form the chelation product (thefluorophore) and finally against the acetate buffer to remove excessEu(III).

The dihydropyridine condensation product chelated to Eu(III) was coatedonto a thin, transparent polystyrene sheet by physical adsorption. Thecoated polystyrene sheet was place on the face of a Radio Shack modelphotovoltaic cell. The photo-conducting device was exposed to sunlight.The electrical current was measured and compared to the electricaloutput monitored from the Radio Shack model with only the polystyrenesheet on its face and under the same conditions.

According to a second aspect of the invention, the formation of thedihydropyridine condensation product is used as a means of detecting thepresence of amines or aldehydes. The detection system is used especiallyin high performance liquid chromatography (HPLC). Thus, a process fordetecting the presence of an amine in a material stream comprisescontacting the material stream with a β-diketone and an aldehyde withchelation functionality carrying a lanthanide metal ion by chelation anddetecting the presence or absence of fluorescence at the excitationmaximum of the dihydropyridine condensation product. Similarly, aprocess for detecting the presence of an aldehyde in a material streamcomprises contacting the material stream with a β-diketone and an aminecarrying a lanthanide metal ion by chelation and detecting the presenceor absence of the fluorescence at an appropriate excitation maximum ofthe dihydropyridine condensation product. The chelate attached to theamine could have a high stability constant for chelating the metal ionas well as being a good donor for example as disclosed in EuropeanPatent Application No. 0,195,413.

It may be advantageous to include a synergist to stabilize and augmentthe lanthanide ion fluorescence, for example trioctylphosphine oxide orother known synergist. It may also be desirable for the contacting to becarried out at a suitably elevated temperature to speed up the formationof a detectable quantity of the condensation product, for example, itcould be carried out as a post-column treatment of the sample in HPLC.

Such a detection method enjoys the sensitivity associated withlanthanide ion fluorescence, especially when time-resolution principlesare used to achieve specificity. The absorption maximum of thedihydropyridine condensation product should of course be different fromthat of the β-diketone starting material, otherwise a separation stepmight be required.

An example of an improved amine detection system would includecontacting an unknown solution with a β-diketone and an aldehydechelated to lanthanide metal ion. The detection of fluorescence at theexcitation maximum of the dihydropyridine condensation product asillustrated in FIG. 7 indicates the presence of an amine.

An example of an improved aldehyde detection system would operate on thesame principles as the amine detection system described above with theexception that the aldehyde reactant is replaced with an amine carryinga lanthanide metal ion by chelation.

The invention is further illustrated by the following nonlimitingexamples:

EXAMPLE 1

Different wavelength-shifting devices were placed in front of asilicon-photovoltaic cell which was then exposed to sunlight. Theelectrical output from the photovoltaic cell was measured in voltsproduced across an electric motor. The results of this experiment areshown below in Table 1. The degree to which the fluorophore enhanced thephotovoltaic cell is recorded in terms of percent. Thesilicon-photovoltaic cell was a Radio Shack model numbered 277-1201.Each wavelength-shifting device included a polystyrene film base. Someof the devices also included coatings of the various fluorophoresdescribed in this paper. The dihydropyridine condensation product isdenoted as DHP in Table 1.

                  TABLE 1                                                         ______________________________________                                        Configuration Voltage Across Motor                                                                         % Increase                                       ______________________________________                                        Plain film    0.411          --                                               Film with DHP Tb.sup.3+                                                                     0.422          2.68                                             Film with DHP Eu.sup.3+                                                                     0.445          8.27                                             ______________________________________                                    

EXAMPLE 2

This is an example of an amine detection system. Proteins arecharacterized by their amine tails. The concentration of proteins in asolution was determined by measuring the Eu(III) ion fluorescenceassociated with the dihydropyridine condensation product. The Eu(III)ion fluoresces at the new excitation wavelength introduced by productformation. The materials used in this example are as follows

TBA - TFA: 0.5×10⁻³ M in a 0.1M acetate buffer at pH 5.5

Chelating aldehyde: 0.37×10⁻³ M in a 0.1M acetate buffer at pH 5.5

Antibody solution: Serially diluted in a 0.1M acetate buffer at pH 5.5

The procedure used in this example is as follows: 0.5 ml each ofaldehyde (with Eu) and β-diketone solution were added to 1 ml aliquotsof different known concentrations of antibody. The solutions wereincubated for one hour at 37° C. and cooled to room temperature.Fluorescence was measured in a Perkin Elmer LS5 spectrofluorometer undera delayed fluorescence mode with the following settings:

Delay 0.05 ms

Gate 0.5 ms

Fixed scale 2.0

Slits Ex 15; Em 20; Ex 277 nm Em 615 nm

The results from this example are shown below in Table 2.

                  TABLE 2                                                         ______________________________________                                        Protein Concentration (mg/ml)                                                                   M         Fluorescence                                      ______________________________________                                        0.87              5.7 × 10.sup.-6                                                                   68                                                0.22              1.4 × 10.sup.-6                                                                   18                                                0.05              0.36      9                                                 0                           4                                                 ______________________________________                                    

The same samples were photon counted using a time-gated fluorometerafter separation of small molecules. The counts measured are recorded inTable 3. The data in Table 3 was used to construct a calibration curvefor determining unknown protein concentrations by interpolation.

                  TABLE 3                                                         ______________________________________                                        Protein Concentration (mg/ml)                                                                      Counts                                                   ______________________________________                                        0.87                 2.2 × 10.sup.8                                     0.22                 5.5 × 10.sup.7                                     0.05                 1.4 × 10.sup.7                                     0.01                 3.0 × 10.sup.6                                     0.003                8.6 × 10.sup.5                                     0                    2.0 × 10.sup.3                                     ______________________________________                                    

EXAMPLE 3

This is an example of an aldehyde detection system. The concentration offormaldehyde, an aldehyde, in a solution was determined by method of thecondensation product formation together with delayed-timegated Eu(III)flourescence measurements similar to Example 2. The materials used inthis example are as follows:

1. A 1×10⁻³ M concentration of β-diketone (TBA-TFA) containing astoichiometric amount of Eu(III) in a 0.1M acetate buffer at pH 5.5

2. A 2.0M concentration of amine - ammonium acetate in a 0.1M acetatebuffer at pH 5.5

3. Formaldehyde solutions diluted in a 0.1M acetate buffer at pH 5.5

The stock solution consisted of a β-diketone solution mixed withammonium acetate solution in a 1:1 ratio. The procedure used in thisexample is as follows:

1 ml of the mixed stock solution of β-diketone and ammonium acetate wereadded to 1 ml aliquots of diluted formaldehyde solutions. The solutionswere incubated for 1 hour at 37° C. and cooled to room temperature. Thefluorescence of the solution was measured in a Perkin ElmerSpectrofluormometer without separation.

The results from this example are as follows: The excitation spectrumdid not have two peaks. It had the identical peak corresponding to theβ-diketone. The peak at 280 nm in the case of the antibody product isattributed to the antibody. The signal detector was necessary toeliminate noise from the unreacted β-diketone-Eu(III) complex. Thereadings therefore decreased with increasing concentration as shownbelow in Table 4. A calibration curve, similar to that constructed inexample 2, could be used to determine the formaldehyde concentrations.

                  TABLE 4                                                         ______________________________________                                        Formaldehyde   Reading Ex 350                                                 Concentration (M)                                                                            Em 615 Scale Factor                                                                          Counts                                          ______________________________________                                        10.sup.-3      168            8.6 × 10.sup.6                            10.sup.-4      61             3.2 × 10.sup.8                            10.sup.-5      204            3.8 × 10.sup.9                            10.sup.-6      251            4.4 × 10.sup.9                            10.sup.-7      256            4.5 × 10.sup.9                            blank          266            4.6 × 10.sup.9                            ______________________________________                                    

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and there is no intention to exclude any equivalencethereof. Hence, it is recognized that various modifications are possiblewhen within the scope of the present invention as claimed.

What is claimed is:
 1. A wavelength-shifting device useful whenoptically coupled to a photovoltaic cell to increase the portion of thesolar spectrum available to the cell for conversion into electricitycomprising the reaction product of a β-diketone-bearing reagent, analdehyde-bearing reagent, and an amine-bearing reagent, and a lanthanidemetal ion, said reaction product chelating the lanthanide metal ion intoa transparent matrix with the chelate being incorporated into the matrixso that the resulting wavelength-shifting device can be opticallycoupled to a photovoltaic cell.
 2. The wavelength-shifting device as setforth in claim 1 wherein the amine is a polymer or copolymer.
 3. Thewavelength-shifting device as set forth in claim 4 wherein the aldehydeis a polymer or copolymer.
 4. The wavelength-shifting device as setforth in claim 1 wherein the β-diketone is a polymer or copolymer. 5.The wavelength-shifting device as set forth in claim 1 wherein thewavelength-shifting device is formed as a transparent polymer orcopolymer.
 6. The wavelength-shifting device as set forth in claim 5wherein the reaction product is dispersed in a transparent plastic. 7.The wavelength-shifting device as set forth in claim 6 wherein thereaction product is a stable 1:1 lanthanide ion chelate fluorophore. 8.The wavelength-shifting device as set forth in claim 5 wherein thereaction product is dispersed in glass.
 9. The wavelength-shiftingdevice as set forth in claim 8 wherein the reaction product is a stable1:1 lanthanide ion chelate fluorophore.
 10. The wavelength-shiftingdevice as set forth in claim 5 wherein the reaction product is a stable1:1 lanthanide ion chelate fluorophore.
 11. A photovoltaic device of thetype comprising a photovoltaic cell having a face wherein theimprovement comprises a wavelength-shifting coating on its face, thewavelength-shifting coating comprising the reaction product of aβ-diketone, an aldehyde, an amine, and a lanthanide metal ion, saidreaction product being a lanthanide metal ion chelate and saidwavelength-shifting coating increasing the portion of the solar spectrumavailable to the cell for conversion into electricity.